What's the meaning of the direction of a highway?

Beginner question: what’s the meaning of the direction of a highway, if any? I couldn’t find any information about that in the wiki.

It doesn’t represent the direction of traffic, since that’s specified by the oneway= tag. Does it represent the direction of growing street numbers / mileage? Or perhaps it doesn’t represent anything in particular, besides defining a reference for tags such as incline= ?

The direction of any way is only relevant where tags such as oneway or incline are present, the meaning of which is based on the way direction. See http://wiki.openstreetmap.org/wiki/Category:Way_Direction_Dependent for a (probably incomplete) collection of tags that take way direction into account.